Method for manufacturing solar cells

ABSTRACT

A method for manufacturing solar cells, more specifically thin-film solar cells, particularly thin-film solar cells on which CuInSe 2  is deposited. A substrate is provided on which a multi-layer structure is formed by depositing thereon a layer defined by a compound including several basic substances, such as copper-indium-diselenide (CuInSe 2 ), or a closely-related compound where copper (Cu) and indium (In) can be replaced totally or partially with Silver (Ag) and gallium (Ga), respectively, and where selenium (Se) can be replaced totally or partially with sulphur (S) and tellurium (Te), and where the concentration of the basic substances in the layer varies. The substrate is placed on the inside of a rotatable, tubular carrier device, after which the substrate is heated, and substance sources are provided for depositing the basic substances on the substrate. One or more of the basic substances is deposited on the substrate by passing respective basic substance sources axially into the tubular carrier device while simultaneously rotating the tubular carrier device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing solar cells,and more specifically thin-film solar cells.

2. Description of the Related Art

The technique used to produce thin-film solar cells is the PVD-technique(Physical Vapor Deposition), in which copper, indium and selenium arevaporized and deposited on a heated substrate. The substrate normallyincludes a glass plate on which a layer of molybdenum (Mo) is sputtered.The compound copper-indium-diselenide (CuInSe2) is formed on thesubstrate. A layer of cadmium sulphide (CdS) and a transparent electriccontact in the form of doped zinc oxide (ZnO) are laid over a layer ofcadmium sulphide (CdS).

Vaporization of the various substances takes place in a chamber. Whenthe glass plate has large dimensions, the plate is positioned stationaryin relation to the sources of the aforesaid substances.

Three problems are primarily encountered when producing thin-film solarcells based on CuInSe2 by means of vacuum deposition (PVD). This alsoapplies with layer compositions other than just CuInSe2.

The first of these problems resides in the ability to produce layers onlarge surfaces in a manner which will provide a uniform layer. Thin-filmsolar cells normally have a size of 1×0.4 meters. The thickness of theCuInSe2-layer is particularly critical and should be uniform in order toachieve good functioning of the solar cell.

The second of the aforesaid problems is that the composition of theCuInSe2-layer should vary over its thickness, so that the composition ofthat part of the layer furthest away from the glass plate will containmore indium than is given by the stoichiometric composition.Correspondingly, that part of the layer which lies nearest the glassplate should contain a relatively higher percentage of copper. This isbecause of the desire to suppress the formation of so-called coppernodules on the upper boundary surface of the layer, since such nodulesreduce the efficiency of the solar cell.

The known technique solves this problem by first applying a layer richin copper and then applying to this first layer in a second processstage a second layer which is rich in indium. Application of a largequantity of indium suppresses the formation of a copper-rich surface,which in turn suppresses the formation of said nodules. This knowntechnique thus requires two different process stages, thereby renderingthe process complicated and expensive.

The third of the aforesaid problems resides in a relatively low yield ofvaporized substances that are deposited on the substrate.

These problems are solved by means of the present invention.

SUMMARY OF THE INVENTION

The present invention thus relates to a method of manufacturing solarcells, more specifically thin-film solar cells, in which a multilayerstructure is built-up on a substrate. The method includes steps ofdepositing a layer of a compound of several basic substances, such ascopper-indium-diselenide (CuInSe2) or a closely related compound inwhich copper (Cu) and indium (In) can be replaced totally or partiallywith silver (Ag) or gallium (Ga) respectively, and where Se is replacedeither totally or partially with sulphur (S) or tellurium (Te), and inwhich the concentration of the basic substances in the layer varies. Themethod is characterized in that the substrate is placed on the inside ofa rotatable, generally circular carrier device; in that the substrate isheated; in that sources of the basic substances to be deposited on thesubstrate are present; in that one or more of the basic substancesis/are deposited on the substrate by passing a respective source axiallythrough the cylindrical carrier device; and in that the cylindricalcarrier device and/or respective sources can be moved while rotating thecylindrical carrier device.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to anexemplifying embodiment thereof illustrated in the accompanying drawing,in which

FIG. 1 is a schematic sectional view of a solar cell;

FIG. 2 is a schematic, axial view of an arrangement for carrying out theinvention; and

FIG. 3 is a schematic side view of the arrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic sectional view of a thin-film solar cell.Thin-film solar cells are constructed on substrates of relatively largesizes, for instance a size of 1×0.4 m. Such structures include a verylarge number of cells above the substrate surface, these cells bringmutually connected electrically. FIG. 1 shows only a part of one suchcell. The technique of manufacturing a large number of mutuallyseparate, but electrically connected cells on a substrate surface iswell known and willnot be described here.

The substrate 1 is normally a glass plate of suitable thickness,generally 3 mm. A layer 2 of molybdenum (Mo) is first sputtered onto theglass surface. The molybdenum layer forms an electric back-contact andpositive pole, or terminal, in the finished cell. The molybdenum layermay have a thickness of, e.g., 0.5 micrometer. There is then applied tothe molybdenum layer a layer 3 of CuInSe2, having a thickness of 2micrometers, for instance. There is then applied onto the layer 3 afirst layer 4 of cadmium sulphide (CdS) to a thickness of from 5 to 500Å for instance, and then an electrical contact in the form of atransparent,doped zinc oxide layer (ZnO) 6 which is applied to athickness of from 1 to2 micrometers, for instance. A metallic contact 5made of aluminium for instance may also be used.

When sunlight falls on the solar cell, an electric voltage will occurbetween the contact 6 (ZnO), which is a minus pole, and the back contact2.

As before mentioned, the present invention relates to a method formanufacturing solar cells, more specifically thin-film solar cells, inwhich a multi-layer structure is built-up on a substrate, said methodcomprising the steps of depositing a layer consisting of a compound ofseveral basic substances, such as copper-indium-diselenide (CuInSe2) ora closely related compound in which copper (Cu) and indium (In) can bereplaced totally or partially with silver (Ag) and gallium (Ga)respectively, and in which Se can be replaced totally or partially withsulphur (S) or tellurium (Te), where the concentration of the basicsubstances in the layer varies.

More specifically, the present invention relates to a method formanufacturing thin-film solar cells in which a layer ofcopper-indium-diselenide (CuInSe2) is deposited and in which theconcentration of copper and indium respectively vary in the layer forthe reasons given in the introduction.

The present invention is described below with reference to anexemplifying embodiment thereof in which a layer of CuInSe2 isdeposited, although it will be understood that the invention is notrestricted to such a layer but can be applied for depositing layerswhich comprise other basic substances.

According to the invention, the substrate 1 is placed on an innersurface 11 of a rotatable, generally cylindrical carrier device 10, asillustratedschematically in FIG. 2. The whole of the carrier device 10is placed in a vacuum chamber 12 (see FIG. 3) of a suitable known kind.The molybdenum layer 2 has been applied to the substrate previous toplacing the substrate on the carrier device 10.

The carrier device 10 may have any suitable construction. For instance,it may comprise a framework of steel beams provided with glass-plate orsubstrate attachment devices.

The arrangement as shown in FIG. 3 includes a selenium source 13 fromwhichselenium is deposited onto the substrate. The position of theselenium source is not critical. Nevertheless, selenium will be presentin sufficient concentration in the chamber atmosphere.

The CuInSe2-layer 3 is deposited, by passing a copper source and anindium source axially through the cylindrical carrier device 10 whilethe device is rotated about its own axis. The sources and the carrierdevice preferably move continuously in relation to one another, with thesubstance sources moving from one open end of the carrier device to itsother end.

Movement of the sources 14, 15 through the carrier device may beeffected by moving the cylindrical carrier device 10 and/or the sources14, 15.

When depositing the basic substances, the sources 13, 14, 15 are heatedso as to generate in the chamber an atmosphere of predeterminedcomposition, in a known manner. The substrate is heated to a temperatureof 300°-600° C.

FIG. 3 illustrates schematically only one conceivable embodiment of themechanical part of the arrangement. The carrier device 10 may beprovided with circular beams 15, a 16 which extend around the peripheryof said device and rest on rollers 17, 18. The rollers 17, 18 may bedriven, for rotation of the carrier device 10. The sources 13, 14, 15may be mounted on a rod 19 which can be moved in relation to a carrierarm 21 in the directions indicated by the arrow 20. For instance, a ballscrew may be fixedly mounted in relation to the carrier arm 21, whichcoacts with a drive unit 22 fixedly connected to the rod 19. The rod ismoveable betweenthe rear position shown in full lines in FIG. 3 and theforward position shown in broken lines 23.

The chamber 12 has a door 24.

The sources may be stationarily mounted instead of being moveable, andthe carrier device mounted for movement in the direction of itsgeometric axis.

Because, in accordance with the invention, the CuInSe2-layer 3 isdepositedby moving a copper source and an indium source at a uniformspeed axially through the carrier device 10 while rotating the carrierdevice 10 about its own axis, a uniform layer will be deposited despitethe sources delivering their respective basic substances, in accordancewith a given space distribution.

According to a much preferred embodiment, the copper source and theindium source are spaced apart at a predetermined distance in theirmovement directions. Furthermore, the copper source 14 is brought intoposition first and then the indium source 15.

Because the copper source and the indium source are placed atpredetermineddistance apart and the copper source 14 is brought intoposition first and then the indium source 15, the amount of copperdeposited on the substratestructure will initially be greater than theamount of indium that is deposited, i.e. a larger amount of coppernearest the substrate, whereafter the amount of indium deposited will begreater than the amount of copper deposited. This means that theCuInSe2-layer 3 will be richer incopper nearest the substrate and richerin indium nearest the CdS-layer 4.

According to one preferred embodiment of the invention, the distancebetween the copper source and the indium source is in the order of 10%to 25% of the radius of the carrier device.

According to a much preferred embodiment, deposition is effected bymoving the copper source and the indium source relative to the carrierdevice solely once in one direction. In the case of the aforesaiddimensions, this means that the sources will move continuously throughthe carrier device over a time period of 30 minutes.

Alternatively, the sources may be moved backwards and forwards severaltimes and the temperatures of respective sources controlled so as toobtain a copper-rich layer nearest the substrate and an indium-richlayer on the opposing surface. It is extremely difficult, however, toachieve sufficient reproducibility with this particular technique.

In that case in which the sources are moved only once through thecarrier device and then returned to their aforesaid withdrawn position,the temperature of the sources is decreased so no substance will bedeposited when the sources are withdrawn through the carrier device,back to their original positions.

According to one preferred embodiment, the cylindrical carrier device isrotated at a speed greater than about 1 to 5 r.p.m.

According to another preferred embodiment, the circumferential innersurface of the cylindrical carrier device is essentially covered withsubstrate, preferably planar substrate, such as to form a polygonalcross-section; see FIG. 2 in which the substrates form an eight-sidedfigure in cross-section. This greatly increases the yield of thequantity of substances delivered by the sources to the quantity ofsubstances deposited on the substrate in comparison with the knowntechnique. The yield is troublesomely low in the case of the knowntechnique.

According to a further preferred embodiment, the length of thecylindrical carrier device corresponds to the length of a solar cellpanel, i.e. about1 meter, and a diameter such that a substrate having awidth of 0.4 meter will form a polygon when seen in cross-section,preferably a hexagonal or decagonal figure.

It will be obvious from the aforegoing that the present invention solvesthe three problems mentioned in the introduction. Consequently, thepresent invention provides an inexpensive and quick method ofmanufacturing thin-film solar cells of very high quality.

The invention, however, is not restricted to the deposition of CuInSe2,butmay also be used in conjunction with other related compounds wherecopper is replaced totally or partially with silver and in which indiumis replaced solely or partially with gallium, and where selenium isreplaced totally or partially with sulphur, or tellurium may be used.

Although the invention has been described with reference to a number ofexemplifying embodiments thereof, it will be obvious that the carrierdevice and also the means for supporting the substance sources may beconstructed in other ways suitable for carrying out the inventivemethod. Furthermore, the sources may be positioned other thanimmediately behind one another. The above-mentioned dimensions andspeeds may also be adaptedto suit prevailing conditions and aretherefore not limited to the aforegiven values.

The present invention is therefore not restricted to the aforedescribedandillustrated exemplifying embodiments thereof, since these embodimentscan be modified within the scope of the following claims.

I claim:
 1. A method for manufacturing thin film solar cells including asubstrate plate on which a multi-layer film of several substances isdeposited by vacuum deposition, said method comprising:a. providing agenerally tubular, rotatable substrate carrier having a longitudinalaxis and an inner peripheral surface for supporting substrate materialsto be substantially uniformly coated with a thin film of the severalsubstances; b. positioning a plural of plates formed from the substratematerial on the inner peripheral surface of the tubular substratecarrier; c. providing a plurality of substance sources containingsubstances to be deposited on the substrate plates, wherein thesubstance sources are spaced from each other along a line that extendsin the direction of the substrate carrier longitudinal axis, to providea desired composition gradient of the respective substances in the filmthickness direction, the composition gradient being substantiallyuniform over the coated surface of the substrate; d. heating thesubstrate materials and the substance sources; e. moving the tubularsubstrate carrier and the substance sources relative to each other in asingle movement in a single, axial direction during a depositionoperation to being the substance sources within the inner peripheralsurface of the tubular substrate carrier; and f. rotating the tubularsubstrate carrier and the substance sources relative to each other todeposit substance source material on an inwardly facing surface of thesubstrate in a substantially uniform layer.
 2. A method according toclaim 1, wherein the substance source include a copper source, an indiumsource, and a selenium source, and wherein a layer ofcopper-indium-diselenide (CuInSe2) is deposited on the substrate, and inwhich the concentration of copper and of indium in said layer aredifferent in a layer thickness direction including the step ofdepositing the CuInSe2-layer by passing the copper source and the indiumsource axially through the tubular substrate carrier.
 3. A methodaccording to claim 2, wherein the copper source is introduced into thetubular substrate carrier first and then the indium source thereafter.4. A method according to claim 1, wherein substantially the entire innerperipheral surface of the tubular substrate carrier is covered withsubstrate material to form a polygon in cross-section.
 5. A methodaccording to claim 2, wherein the copper source and the indium sourceare moved linearly relative to the tubular substrate carrier only onceand in one direction.
 6. A method according to claim 1, wherein thetubular substrate carrier is rotated at a speed greater than about 1r.p.m.
 7. A method according to claim 1, wherein the tubular substratecarrier has an axial length corresponding to a length of a solar cellpanel, and a diameter such that a plurality of solar cell panels form apolygon when the carrier is viewed in cross-section, the polygon havingat least six sides.
 8. A method according to claim 7, wherein thedistance between the copper source and the indium source is from about10% to about 25% of the radius of the tubular substrate carrier.
 9. Amethod according to claim 1 wherein the relative rotation of the tubularsubstrate carrier relative to the substrate sources and the relativelinear movement between the tubular substrate carrier and the substancesources are performed simultaneously.